Avionics News July 2013 - 34

ONBOARD TRAFFIC SYSTEMS
Continued from page 33

be possible, but you will need to
budget additional time for dealing
with the FAA. But take note: installation of any antenna on a pressurized aircraft requires engineering
approval.
The manufacturers we spoke with
were unanimous on one key point:
Antenna location is the single most
significant issue in the success or
failure of any onboard traffic system.
It’s critical to study the installation
manual, which will likely specify
that the directional antenna usually
mounted on top of the aircraft must
be a certain distance from other antennas, not be mounted at a significant angle and usually as far forward

avionics news

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july

2013

as possible to avoid shadowing by the
vertical fin. According to Bill Stone
of Garmin, “Traffic systems, by their
very nature, are attempting to process extremely small signals, using
antenna elements just inches apart.
It’s an extremely challenging task –
antenna placement is critical. If it’s
mounted very nicely to the side of the
vertical stabilizer, it won’t work – I
don’t care how nice it looks.”
Avidyne’s Aaron Hall agreed,
and added that in some installations, creativity may be needed. “On
helicopters, you may need to reverse
the install and put dual blades on top
(either side of the rotor hub), and
a single blade on the bottom,” Hall
said. “If you have a DME, it may
need to be diode isolated.”
A group of engineers from

Rockwell-Collins, on the other
hand, said they’ve had good results
with directional bottom antennas on
helicopters, which can compensate
for the inevitable shadowing of a top
antenna by the rotor hub.
Another major installation issue
is cabling – cables must be cut to
exactly equal length. Failing to do
so can lead to traffic appearing in
the wrong position or intermittent
failure of the system. RockwellCollins suggested shops that rarely
do a TCAS installation subcontract
with specialists to have harnesses
fabricated, and Avidyne’s Hall
warned that loose cable connections
can lead to overheating and damage
to the system processor. Another
issue is the need for an adequate
ground plane – usually not a problem in metal airplanes, but when a
composite airplane is encountered,
some extra effort may be required.
Both Avidyne and Garmin suggested
that copper foil tape can be used, but
in these and any other special cases,
the manufacturers all recommend
shops call for advice.
Honeywell’s Scott Miller said
training is an issue: “Make sure the
operator flies away understanding
how the system works – in particular, the cursor control device. You
don’t want to be fumbling around
with a cursor in congested airspace.”
Finally, here’s a useful tip on
troubleshooting. According to
Garmin’s Stone, “Modern systems
– ours and others – have diagnostic
and recording capability. You can
retrieve logged data, which we can
review and assist with installation
and troubleshooting issues.” He
added that in some cases, it enables
them to identify a loose cable, or
even which pin may be loose on a
connector.
This sure sounds easier than the
“half-split” troubleshooting technique I was taught in the Ohio Air
National Guard. q

http://www.sensorantennas.com

Avionics News July 2013

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